FIG. 5 and FIG. 6 are an oblique diagram and a sectional diagram, respectively, showing an example of an overall arrangement of a reflective large-aperture optical or infrared telescope apparatus. In the figures, 1 is a parallel light beam, having arrived from an observed celestial body; 2 is a primary reflective mirror, which collects the light beam; 3 is a secondary reflective mirror; and 4 is a Cassegrain focal point, at which an observation device is placed. 5 is a telescope tube assembly which supports secondary reflective mirror 3 and primary reflective mirror 2 and which swivels around axis E.sub.1. The telescope tube assembly 5 is comprised of: top ring 5b, which supports secondary reflective mirror 3; mirror cell 5a, which supports primary reflective mirror 2; and center section 5e, which supports these through structural members 5d and 5c. 6 is a mount structure which has a function of rotating telescope tube assembly 5 around axis E.sub.1 and which itself freely rotates around axis A.sub.z while holding telescope tube assembly 5.
FIG. 3 and FIG. 4 are sectional diagrams showing details of structural members 5c and 5d of mirror frame 5; a structural member provides rigidity and strength to support the weight of top ring 5b and mirror cell 5a and is formed, for example, of a steel tube 5c.sub.1 or 5d.sub.1 coated on the outside with rust-preventing agent 5c.sub.2 or 5d.sub.2. In place of steel tube 5c.sub.1 or 5d.sub.1, some structural members utilize tubes 5'c.sub.1 or 5'd.sub.1, which are made of a material having a low coefficient of thermal expansion, such as CFRP (Carbon Fiber Reinforced Plastic) or Invar, as shown in FIG. 3.
When a celestial body is to be observed, the observation slit (not shown) in the dome is opened, the telescope apparatus is set to point in the direction of that celestial body by rotation about the E.sub.1 axis of the mirror mount and furthermore by rotation to change the angle A.sub.z. Observation is made at the position of Cassegrain focal point 4, where an observation device, for example, a photographic dry plate, is accurately positioned.
When the dome observation slit is opened, the temperature of the air around the telescope apparatus varies depending on the temperature change of the outer air due to air exchanged through the observation slit. Furthermore, in a case of optical or infrared telescope, observation is made almost always at night; the telescope is in an environment in which radiative cooling from the cold upper layers of the atmosphere begins instantly when the observation slit is opened. Consequently, structural members 5c and 5d are exposed, simultaneous to the start of observation, to a fluctuating ambient temperature and radiative cooling from the cold air of the upper atmosphere.
As the prior-art structural members had a structure shown in FIG. 3 or FIG. 4, there were the following problems.
(1) Heat exchange with the outer air is performed through convection; however, since the heat capacity of the structural members overall is large, the structural member is slow in following the outside air (ambient) temperature variations; and there tends to occur a difference in temperature from the ambient air. When such a difference in temperature occurs, air becomes turbulent due to convection; and, since the refractive index of light becomes non-uniform, observed light is disturbed. This phenomenon is called seeing deterioration.
(2) Since the emissivity of rust-preventing agent coated on the surface of the structural member is high, the temperature of the structural member is strongly affected by radiative cooling from the upper atmosphere and becomes less than ambient air temperature; a temperature difference occurs; and, in this case as well, seeing deterioration occurs.
(3) The temperature of a structural member varies during observation; therefore, thermal expansion or contraction occurs in the length direction; and because of this the distance between primary reflective mirror 2 and secondary reflective mirror 3 varies, the position of Cassegrain focal point 4 varies, and observation is adversely affected. Also, although this defect can be eliminated by the use of a low thermal expansion material for the structural member material as shown in FIG. 3, this is expensive, and furthermore problem at (1) and (2) above still remain.